The present invention relates generally to the field of optical signal processing apparatus, and more specifically to a micromirror spatial light modulator of simplified construction and improved performance.
Spatial light modulators are device which allow control of an optical wavefront for processing or imaging operations. These devices, often referred to as light valves in the literature, have potential for application in large screen display systems as well as in optical data processing systems, including missile guidance and robotic vision systems.
The strong interest in large screen display systems as a substitute for CRT projection systems has led to the development of various forms of spatial light modulators based upon the presence of micromechanical reflective elements, such as deformable or deflectable micromirror elements. Listed below are several publications which describe their construction and operation.
(A) H. C. Nathanson and J. R. Davis, Jr., "Electrostatically Deflectable Light Valves for Projection Displays", U.S. Pat. No. 3,746,911, issued July 17, 1973.
(B) J. Guldberg, H. C. Nathanson, D. L. Balthis and A. S. Jensen, "An Aluminum/SiO.sub.2 /Silicon-on-Sapphire Light Valve Matrix for Projection Displays", Applied Physics Letters, Vol. 26, No. 7, pp 391-393, 1975.
(C) R. N. Thomas, J. Guldberg, H. C. Nathanson and P. R. Malmberg, "The Mirror-Matrix Tube: A Novel Light Valve for Projection Displays", IEEE Transactions on Electron Devices, Vol. ED-22, No. 9, pp 765-775, 1975.
(D) K. E. Peterson, "Micromechanical Light Modulator Array Fabricated on Silicon", Applied Physics Letters, Vol. 31, No. 8, pp 521-523, 1977.
(E) K. E. Peterson, "Dynamic Micromechanics on Silicon: Techniques and Devices", IEEE Transactions on Electron Devices, Vol. ED-25, No. 10, pp 1241-1250, 1978.
(F) B. H. Soffer, D. Boswell, and A. M. Lackner, "Optical Computing With Variable Grating Mode Liquid Crystal Devices", SPIE Vol. 232, pp 128-132, 1980.
(G) R. E. Brooks, "Micromechanical Light Modulators for Data Transfer and Processing", SPIE, Vol. 465, pp 46-54, 1984.
(H) L. J. Hornbeck, "128.times.128 Deformable Mirror Devices", IEEE Transactions on Electron Devices, Vol. ED-30, No. 5, pp 539-545, 1983.
(I) D. R. Pape and L. J. Hornbeck, "Characteristics of the Deformable Mirror Device for Optical Information Processing", SPIE, Vol. 388, pp 65-74, 1983.
Briefly, a micromechanical spatial light modulator may comprise a micromirror array, where the micromirror elements are deflected by electrostatic forces, in close proximity to an electrical addressing system. The addressing scheme may consist of simple metallic electrodes, or a vacuum tube electron beam system, which could be a microchannel plate structure. An electrical charge is applied to selected elements of the micromirror array by the addressing scheme and these mirror elements deflect. The readout illumination applied to the micromirror elements is in turn deflected in accordance with the deflection of the micromirror elements. Readout imaging is achieved by means of a reflective Schlieren optical arrangement. Diffractive readout is also possible, which is of interest in holography or phase conjugation applications.
The details of the addressing scheme are a difficulty encountered in the practical implementation of such micromirror spatial light modulators. A simple electrode structure can address only an insignificant number of micromirror elements whereas imaging or optical processing applications require upwards of one thousand mirror elements.
Electron beam addressing has proved successful, as demonstrated in the Guldberg et al and Thomas et al publications (B and C) supra. However this type of addressing has the disadvantage of beam scanning, which limits the write speed, and the further disadvantage of using vacuum tube technology, which adds to the bulk and weight of the device. Vacuum operation also eliminates air damping and hence the settling time of the mirror elements is increased.
Intensity-to-position encoding can be achieved with a variable grating liquid crystal device as described in the Soffer et al publication, (F) supra. However, the slow response of the device limits its applications.